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Abstract :
[en] At the end of the Devonian, several profound extinctions affected a large number of marine groups.
However, some of them, such as holocephalan chondrichtyans, showed a great diversification
during the recovery of the ecosystems, during the Tournaisian. Despite the fact that a large taxic
diversity has been documented for these holocephalans; their ecological diversity is however poorly
known, because the shape of isolated teeth can be a poor predictor of the ecology of these animals.
Microwear analysis has the potential to reveal distinct diets and actual use of teeth in these extinct
animals during the Tournaisian. We analysed the microwear of Tournaisian holocephalans from the
Tournai and Ourthe formations of Belgium. Dental microwears were observed qualitatively on 20
teeth with a scanning electron microscope and mapped and analysed in detail for 7 of them with
ArcMap software. While pits are almost totally absent in our sample, our microwear dataset
revealed two populations of scratches with distinct length distributions. We suggest that these
populations were produced by two different mechanisms. The first population contains mainly long
scratches (>0.2 mm, up to 2.0 mm) that are often oriented 40° to 70° compared to the
anteroposterior axis of the tooth. We propose that these scratches would have been produced by
trituration. The second population comprises almost exclusively of short scratches (<0.2 mm)
especially abundant on the mesial face of the teeth and preferentially oriented subparallel to the
anteroposterior axis. They would have been produced when the holocephalans dug into sea bottom
sediments while searching for food. To identify materials that might have caused the observed
microwear, we compared the hardness of the holocephalan orthodentine, making the bulk of the
crown of holocephalan teeth, and materials present in their environment. The skeleton of a wide
series of marine organisms (crinoids, brachiopods, molluscs) is composed of calcite or aragonite,
which appears to be slightly harder than holocephalan orthodentine. These materials may thus
scratch holocephalan teeth but are hardly able to produce pits because of the small difference in
hardness. Tournaisian holocephalans were thus probably feeding on benthic faunae and they likely
dug in the sediment at the search of food. If correct, this might rule out prey items located clearly
above the sea floor, such as ammonioids or high-stalked crinoids. However, most of our specimens
showed similar microwear features, which prevents us to highlight ecological differences between
the taxa we sampled.
